One effective method for inspecting and monitoring various structures is guided wave testing (GWT) using magnetostrictive sensor (MsS) technology. The principle of magnetostriction is based on either shifting or oscillation/rotation between magnetic domains in the material due to applied magnetic fields. Typically, a permanent magnetic field is used to give the domains a preferred orientation. Variable magnetic fields are also applied to initiate the rotation of the domains causing the dimensional changes. Depending on the mutual orientation (in-plane or out-of-plane) and on the mutual magnitude of the magnetic fields, oscillation of domains can produce longitudinal or transverse vibrations.
There are many applications of MsS testing, with testing of pipelines being a common application. Pipeline testing applications often use MsS transducers placed around the outside of the pipeline.
In contrast to testing pipelines, for which access to the inside of the pipeline may be impractical, testing other types of tubular structures may be best performed with an MsS transducer placed inside the tubular structure. For example, testing heat exchanger tubes is often performed using an MsS probe inserted into an open end of the tubing.
If the generated MsS waves are coupled to the inside of the tube, the waves propagate along the tube and are partially reflected by geometric irregularities present in the tube, such as corrosion defects. The reflected signals are then detected by the MsS transducer. From the arrival time of the reflected signal and the signal amplitude, the axial location of the irregularity and its severity are determined.